1. Field of the Invention
The present invention relates to a waste heat recovery apparatus for the recovery of waste heat of an exhaust gas discharged from a high temperature furnace, such as an industrial furnace.
2. Statement of the Related Art
Conventionally, in order to improve the thermal efficiency of various furnaces, use has been made of heat exchangers for recovering the waste heat of high temperature exhaust gas discharged from the furnace. The recovered waste heat is used, for example, to pre-heat fresh combustion air to be supplied to the furnace. The heat exchanger used for this purpose is generally provided with metallic radiation tubes having a limited heat resistivity. Thus, when the exhaust gas temperature exceeds 800.degree. C., for example, the gas cannot be supplied to the heat exchanger directly because of the insufficient heat resistivity of the metallic radiation tubes.
It has therefore been considered necessary to add a substantial amount of cooling air, or other suitable cooling gas, to the exhaust gas before the latter is supplied to the heat exchanger, to thereby lower the exhaust gas temperature below 800.degree. C. With the addition of the cooling gas, however, exhaust gas post-treating equipment, such as a scrubber, connected on the downstream side of the heat exchanger and used to remove harmful SOx and NOx compositions contained in the exhaust gas, is charged with an increased amount of gas. The capacity of the post-treating equipment has to be increased, and this is disadvantageous from an economical viewpoint. Moreover, in the above-mentioned arrangement of a known waste heat recovery apparatus, the temperature of the exhaust gas discharged from the heat exchanger remains as high as 500.degree.-600.degree. C., even after the recovery of the waste heat by the heat exchanger. The heat recovery rate thus amounts to only 40-50%, so that an improvement in the overall thermal efficiency of the furnace can be achieved only in a limited range. This is due to the fact that a substantial amount of the holding heat of the exhaust gas is wasted as heat added to the cooling gas, and cannot be recovered efficiently.
As an alternative approach, it is known to make use of a heat exchanger with radiation tubes made of a ceramic material, through which the exhaust gas at a temperature above 800.degree. C. can be passed directly, without requiring addition of a cooling gas. However, such a heat exchanger is very expensive and the cost is, for example, 5-10 times higher than that of a conventional metallic radiation tube-type heat exchanger. This approach, too, is quite disadvantageous from an economical viewpoint.
With the above-mentioned prior technology, therefore, it has been considered extremely difficult or almost impossible to efficiently and economically recover the waste heat of high temperature exhaust gas at a temperature as high as 1,000.degree. C. or more.